As pointed out in U.S. Pat. No. 3,252,652 and other literature which may be found in the file of the above mentioned copending application, it is possible to produce a high-vacuum by the attachment of gas molecules to a deep cooled surface which can be termed a cryosurface, the apparatus for this purpose being referred to as a cryopump.
The surface is cooled by contacting it with liquid helium, e.g. on a side thereof opposite that at which attachment of the gas molecules will occur.
When a cryopump of conventional design and high gas-removal capacity is used to generate high vacuums, various problems are encountered.
Cryopumps do not remove the pumped gas from the receptacle system to be evacuated but rather freeze out the gas by condensation and/or suction on the deep-cooled surface or cryosurface. Combustible gases, when these are to be abstracted, for example hydrogen, create the possibility that explosive mixtures will develop. This is the case especially when a highly efficient cryopump removes a large quantity of gas from the atmosphere in the vacuum system and thus a large volume of the gas is collected at the pump surface and must be vented. This venting is desirable to renew access to the cryosurface and may even occur spontaneously at undesired times because of operating failures or oversights.
It is essential to remove the condensed layer from time to time in any event because, with increasing thickness of the condensate layer, there is an increase in emissivity and an associated increase in the coolant demand at the cryosurface.
Furthermore, when the gas includes a radioactive isotope of hydrogen, tritium, as is the case in fusion experiments, relatively large quantities of this radionuclide (0.1 to 1 kg.) can collect on the cryosurface and not only create the danger of explosion but also the danger that any sudden or spontaneous release of the collected gas may liberate large quantities of a radioactive and hence dangerous substance.
These problems can be sharply reduced or avoided when the deep-cooled surface of the cryopump is frequently thawed, i.e. freed from the condensate, and the liberated gas and comparatively high pressure and high throughput is discharged by a pump of relatively low suction capacity, e.g. a turbomolecular pump and fed to a processing station.
This requires warming of the cryosurface or the condensation surface by temporarily interrupting the contact therewith with the liquid helium and thus by emptying the liquid helium from the vessel of which this surface forms a part. This requires discharging the liquid helium from the vessel and the usual ducts and supply containers which are in vacuum-tight connection with the receptacle. When the cryosurface is again to be cooled, the liquid helium must be returned.
This two-fold transfer of large amounts of liquid helium between the pump and an external reservoir is time-consuming and generally is accompanied by large liquid helium losses.